RF sputtered Ta2O5 films (23-26 nm) on Si, before and after high temperature (873, 1123 K) O-2 annealing have been investigated with respect to their dielectric and microstructural properties. Both high resolution transmission electron microscopy (HR-TEM) analysis and electrical measurements indicated the presence of extremely thin SiO2 at the interface with Si. The substrate temperature during deposition has no effect on the flatness of the SiO2-Ta2O5 interface but it affects the flatness of the Si-SiO2 interface-for films obtained on heated substrate, this interface becomes abrupt. The as-deposited and the annealed, at 873 K, films are amorphous where as after 1123 K annealing, they crystallize in orthorhombic phase, (there is evidence that the crystal phase electrically manifests as slow states). The crystallization of the films favor a larger dielectric constant at 1 MHz and a smaller leakage current especially in the dielectric voltage region, which will be the operating voltage for future technology generations. A leakage current of similar to10(-9) A/cm(2) at 1 V can be achieved. The results are discussed in terms of relative impact of two concurrent mechanisms during high temperature O-2 treatment-an appearance of crystal phase and a real annealing of the films accompanied with an improvement of the dielectric properties. (C) 2003 Elsevier Science B.V. All rights reserved.